Abstract: This work details the development of a three-dimensional (3D) electric field
model for the LUX detector. The detector took data during two periods of
searching for weakly interacting massive particle (WIMP) searches. After the
first period completed, a time-varying non-uniform negative charge developed in
the polytetrafluoroethylene (PTFE) panels that define the radial boundary of
the detector's active volume. This caused electric field variations in the
detector in time, depth and azimuth, generating an electrostatic
radially-inward force on electrons on their way upward to the liquid surface.
To map this behavior, 3D electric field maps of the detector's active volume
were built on a monthly basis. This was done by fitting a model built in COMSOL
Multiphysics to the uniformly distributed calibration data that were collected
on a regular basis. The modeled average PTFE charge density increased over the
course of the exposure from -3.6 to $-5.5~\mu$C/m$^2$. From our studies, we
deduce that the electric field magnitude varied while the mean value of the
field of $\sim200$~V/cm remained constant throughout the exposure. The varying
electric fields and their impact on event reconstruction and discrimination
were successfully modeled.